Download Rodent abundance, stone bund density and its effects on crop

Meheretu Yonas, Sluydts, V., Kiros Welegerima, Bauer,
H., Mekonen Teferi, Gidey Yirga, Mulungu, L., Mitiku
Haile, Nyssen, J., Deckers, J., Makundi, R., Leirs, H.,
2014. Rodent abundance, stone bund density and its
effects on crop damage in the Tigray highlands, Ethiopia.
Crop Protection, 55: 61-67.
Rodent abundance, stone bund density and its effects on crop damage in the Tigray
highlands, Ethiopia
Yonas Meheretu a.b, Vincent Sluydtsc, Kiros Welegerimaa, Hans Bauerd, Mekonen
Teferia, Gidey Yirgaa, Loth Mulungue, Mitiku Hailef, Jan Nysseng, Jozef Deckers h,
Rhodes Makundie, Herwig Leirsb
a
Mekelle University, Department of Biology, P.o.box 3102, Mekelle, Ethiopia.
b
University of Antwerp, Evolutionary Ecology Group, Groenenborgerlaan 171, Antwerp
2020, Belgium.
c
, Nationalestraat 155, 2000 Antwerp, Belgium.
d
Department of Earth and Environmental Sciences, Catholic University of Leuven,
Celestijnenlaan 200E, B-3001 Heverlee, Belgium.
e
Sokoine University of Agriculture, Pest Management Center, P.o.box 3110, Morogoro,
Tanzania.
f
Department of Land Resources Management and Environmental Protection, Mekelle
University, P.0.box 231, Mekelle, Ethiopia.
g
Department of Geography, Ghent University, B-9000 Gent, Belgium.
h
Division of Soil and Water Management, K.U. Leuven, Celestijnenlaan 200E, 3001
Heverlee, Belgium.
1
Institution where the work is carried out:
University of Antwerp
Department of Biology
Evolutionary Ecology Group
Groenenborgerlaan 171
Antwerp 2020, Belgium.
Corresponding author:
Yonas Meheretu
Present address: Department of Biology, Mekelle University, P.o.box 3102, Mekelle,
Ethiopia. E-mail - [email protected], Tel. +251 914721856.
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Abstract
In areas of subsistence agriculture, a variety of soil conservation methods have been
implemented in the last few decades to improve crop yields, however these can have
unintended consequences such as providing habitat for rodent pests. We studied rodent
population dynamics and estimated crop damage in high and low stone bund density
fields for four cropping seasons in Tigray highlands, northern Ethiopia. Stone bunds are
physical structures for soil and water conservation, and potentially habitat for rodents.
We used a general model to relate the proportion of crop damage to rodent abundance,
stone bund density and crop stages. Generally, rodent abundance remained relatively low
during the study period, except during the fourth quarter of the 2010 cropping season. We
found a positive correlation between rodent abundance and crop damage, and significant
variation in rodent abundance and crop damage between high and low stone bund density
fields. Furthermore, crop damage also varied significantly between crop stages. We
concluded that Mastomys awashensis (Lavrenchenko, Likhnova and Baskevich 1998) and
Arvicanthis dembeensis (Ruppel 1842) were the two most important crop pests in Tigray
highlands causing significant damage. Fields with high stone bund density (~10m
average distance apart) harbor more rodents and endure a significantly higher proportion
of crop damage compared to fields with lower stone bund density (~15m average distance
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apart). The fact that rodent abundances peaked during the reproductive stage of the crop
and around harvest implies the need for management intervention before these crop
stages are attained.
Key words: rodent dynamics; crop damage; stone bund; pest control; Ethiopian
highlands.
1.
Introduction
Approximately 50% of the Tigray province, Northern Ethiopia, is classified as highland
(>2500m a.s.l), characterized by rugged geomorphic features and steep slopes with
narrow intermountain valleys (Vancampenhout et al. 2006). The population of the
highland has grown quickly in the last few decades, at a growth rate of 2.5% per year
with an average family size of five persons per household (CSA, 2008), but the livelihood
of rural families depends on small scale subsistence agriculture (Pender and
Gebremedhin, 2007). Crop production is predominantly rainfed with little irrigation. The
most important constraints to crop production in the highlands are soil erosion and
fertility loss, erratic rainfall, low cereal yield and pre- and post-harvest crop losses to
pests (Woldehanna, 2002; Lemenih et al. 2005; Vancampenhou et al. 2006; Pender and
Gebremedhin, 2007).
In Ethiopia, estimates indicate 15-40% pre-harvest loss due to pests in field crops
(e.g. cereals, pulses and oil seed), 13-29% loss in horticultural crops (e.g. root crops), 948% loss in coffee and 21-60% loss in cotton annually (Amera and Abate, 2008). Other
estimates show that pre- and post-harvest losses to insects, diseases, weeds and vertebrate
pests add up to 30-40% (Abesha, 2006). As in most of the Sub-Saharan African countries,
insect pests are a major agricultural concern in Ethiopia. Migratory insects, such as the
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African armyworm (Spodoptera exempta) and regular pests, such as the Russian wheat
aphid (Diuraphis noxia), occur frequently and result in significant yield losses (Abate,
2006; Belay and Stauffer, 2007). The most common vertebrate pests are the red-billed
quelea (Quelea quelea) and several species of rodents.
In Ethiopia, approximately 84 species of rodents have been recorded; a dozen of
which are considered agricultural pests (Bekele et al. 2003). The most common pest
rodents with widespread distribution in the country belong to two genera: Mastomys
(Thomas) and Arvicanthis (Lesson) (Bekele et al. 2003). Farmers in central (Makundi et
al. 2003) and Northern Ethiopia (Meheretu et al. 2010) have ranked rodents as the
number one pre- and post-harvest crop pests. Bekele et al. (2003), for example, reported
26.4% yield loss of maize crops in the fields due to rodent attacks in central Ethiopia. In
northern Ethiopia, surveyed farmers estimated 9-44% pre-harvest yield loss in annual
production to cereal crops due to rodent attacks (Meheretu et al. 2010).
Farmers and experts in the Tigray highlands have also become increasingly
concerned that some of the local methods used to combat soil erosion and fertility loss are
in fact promoting rodent pests (Gebremichael and Herweg, 2000; Beshah, 2003; Nyssen
et al. 2001, 2007; Meheretu et al. 2010). Massive soil and water conservation programs
focusing on crop fields have been initiated in Tigray in recent decades, and one of these
methods, the building of stone bunds, is of particular concern. Stone bunds are rock walls
built from large basaltic or limestone rock fragments, reinforced by gravel and soil to
reduce holes/gaps between the stones (Nyssen et al. 2001). They are built following the
contours of the topography, with an average height of approximately 1m. In general, the
morphology (height, width and length) of the stone bunds in crop fields is influenced by
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factors such as type of topography (e.g. slope, gully), size of neighboring farms, and
amount of rock fragments in the filed (Nyssen et al. 2001). The stone bunds are also used
to demarcate individual crop fields. The stone bunds therefore potentially provide
extensive and continuous suitable refugia for rodents within cropping areas, and there are
concerns that high stone bund densities in crop fields are associated with high rodent
abundance, leading ultimately to more crop damage (Gebremichael and Herweg, 2000;
Beshah, 2003; Nyssen et al. 2001, 2007; Meheretu et al. 2010).
Yet despite widespread reports of significant crop damage by rodents in Tigray
(and in Ethiopia at large), little is known about the ecology and population dynamics of
the rodent species. Moreover, empirical estimates of crop damage and yield loss due to
rodents are scarce. Nevertheless, knowledge of the relationship between pest population
dynamics, farming techniques and crop damage and the factors contributing to these
relationships are essential to predict future rodent population dynamics and subsequent
crop damage, and to therefore devise a plan for sustainable management (Leirs, 2003;
Singleton et al. 2005; Witmer, 2007).
The objectives of this study were therefore to investigate the temporal dynamics
of rodent populations in rainfed crop fields in the Tigray highlands, and to relate this to
stone bund density and the level of pre-harvest damage and loss to mixed barley and
wheat crops. Furthermore, we report the effects of changes in crop developmental stages
on rodent population dynamics. We predicted (i) crop damage to be positively associated
with rodent abundance, (ii) rodent abundance to be positively associated with stone bund
density and (iii) crop developmental stage to have non-linear effect on rodent abundance.
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2.
Methods
2.1. Study area
The study was conducted in four rainfed crop fields in the May Zeg-Zeg catchment (~200
ha) near the town of Hagere Selam (13°40′N, 39°10′E), Northern Ethiopia, from April
2007 to February 2011 (Fig. 1). The altitude of the study area is about 2600 m a.s.l. and
the morphology of the Hagere Selam area is typical for the Tigray highlands (see Nyssen
et al. 2010 for a detailed description). The area has an annual average rainfall of 762 mm
(as reported for 1970-2005 by Nyssen et al. 2010) and the main rainy season runs from
June to September. Crop production depends on this rain as little irrigation is practiced
and cropped fields are the dominant land use (about 65%) in the study area. The typical
land use is crop fields in the flat areas and lesser slopes and rangeland and exclosures
(guarded areas where grazing and farming are not allowed) on the steep slopes. The
remaining native vegetation is largely dominated by Acacia etbaica and Euclea
schimperi.
The experimental grids were situated on a basaltic Vertic Cambisol soil, where
stone bunds were built in the last two decades to prevent soil erosion (Nyssen et al.
2008). The main crops grown were wheat (Triticum sp.), barley (Hordeum vulgare), a
mixture of wheat and barley, and teff (Eragrostis tef); these are staple crops in the
highlands. Cereal grains, such as wheat and barely, are sown after the early rains in June;
crops reach milky stage in August, mature in October and are harvested in November.
Other commonly cultivated crops include grass pea (Lathyrus sativus), horse bean (Vicia
faba) and lentil (Lens culinaris). Rainfall data for Hagere Selam was obtained from the
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National Meteorological Agency; the Hagere Selam weather station is approximately 2
km from the study area.
Fig. 1
2.2. Grid setup
Four permanent square grids (60 x 60 m) were set in four mixed barley and wheat crop
fields, situated more than 200m apart. Two of the grids represented fields with low stone
bund density (LSBD) and the other two represented fields with high stone bund density
(HSBD). We defined LSBD grids as those with stone bunds spaced ~15m average
distance apart while HSBD grids had stone bunds spaced ~10m apart. All farming
practices were conducted according to the conventional farming system followed by the
farmers in the area. Crop variety and agronomic practices were kept the same
(synchronized) in each grid each year.
2.3. Rodent trapping
A Capture–Mark–Release (CMR) technique was used to study the population dynamics
of the rodent species. Each grid consisted of seven parallel lines, 10 m apart, with
trapping stations also 10 m apart (i.e. a total of 49 trapping stations per grid). Trapping
was conducted with Sherman LFA live traps (7.5 x 9.0 x 23.0 cm, HB Sherman Trap Inc,
Tallahassee, USA) baited with peanut butter. In each grid, trapping was conducted
simultaneously for three consecutive nights every fourth week. Traps were checked early
in the morning and captures were marked by toe clipping and released at the point of
capture (following the ethical policies and guidelines approved by the committee for
Animal Care and Use (Mekelle University).
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2.3. Damage estimation
At milky and maturation stages (about a week before harvest), we surveyed the grids in
order to visualize the distribution of rodent damage. Sections of the grids with relatively
similar damage intensity were grouped in strata as low, medium or heavy damage based
on the ratings of 0–25, 26–50, and >50% damage, respectively, and the proportion of
each rating within each stratum was determined by averaging the visual estimates of two
independent assessors. Then, within each stratum, the number of cut and uncut stems was
counted in a quadrat of 50 x 50cm. Fifteen quadrats were sampled per grid, and the
proportion of quadrats sampled per damage stratum mirrored the proportion of the grid
within each stratum. Stratified quadrat sampling technique was preferred over other
commonly used sampling techniques, such as systematic row sampling technique. In the
Tigray highlands, crop seeding is conducted by broadcasting (i.e. not in rows) and
stratified sampling method is recommended where rodent damage does not appear to be
random (Aplin et al. 2003; Mulungu et al. 2007).
Damage by rodents was distinguished by the characteristic oblique cut through the
stems near the base. The proportion of stems cut (proportion of damage) was calculated
from ((the number of stems cut / the total number of stems (cut and uncut)) X 100). The
mean proportional damage was calculated for the whole grid based on the proportion of
damage for each stratum. For crop loss estimation, 15 panicles were randomly cut from
each quadrat at maturation stages. Weights of seeds per panicle were estimated and
moisture content was measured for a sample of grains from each quadrat.
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2.5. Statistics
Rodent population abundance was estimated from the 3-day CMR trapping sessions using
the m(h) estimator of the Program CAPTURE (White et al. 1982). This estimator
assumes sampling from a closed population during a trapping session and allows for
individual variations in probability of capture (heterogeneity model). This model is
commonly used to estimate abundance in rodent populations and appears quite robust
(Parmenter et al. 2003).
A generalized linear mixed model was fitted to the data to relate the observed
variation in crop damage (the response variable, proportion of damage measured) to
rodent abundance (m(h), continuous), different crop stages (2 levels: milky and
maturation), stone bund density (2 levels: LSBD and HSBD), and year. A logit link
function was used to properly model the proportion of damage observed and a binomial
distribution was assumed to assess statistical inference. Interannual variation was taken
into account by considering factor year as a random effect in the statistical model. Model
selection was based on AIC and likelihood ratio test (LRT) (LRT was used to verify
whether the random effect year should be incorporated into the model, while AIC was
used to make a selection of the different variables to model) and we used a top-down
protocol as described in Zuur et al. (2009). We used the statistical software R-2.13.0 (R
Development Core Team, 2010) and the statistical package lme4 (Bates and Maechler,
2010).
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3.
Results
3.1. Species composition, rodent abundance and density of stone bunds
A total of 1,365 small mammals belonging to at least four species of rodents and one
insectivore (Soricidae) were captured in a total of 27,636 trap nights (Table 1). About
64% of the small mammals were captured in the HSBD grids, which was significantly
higher than in the LSBD grids (X2 = 149.102, d.f = 4, p < 0.0001). The multimammate rat
Mastomys awashensis and the grass rat Arvicantis dembeensis were the two dominant
rodent species, accounting for 87.1% of the captures. While the monthly abundance of M.
awashensis was higher than that of A. dembeensis in LSBD grids throughout the trapping
period (Fig. 2a), the abundance of the two species varied little in HSBD grids, except
from November 2010 to February 2011 when there was an outbreak of both species, and
especially of A. dembeensis (Fig. 2b). However, only the proportion of A. dembeensis
was significantly higher in the HSBD grids than LSBD grids (X2 = 72.5767, df = 1, p <
0.0001). Trap success ((total number of animals trapped / total number of trap night) X
100) was 6.4% in HSBD grids and 3.5% in LSBD grids. The other rodent species trapped
were the spiny mouse Acomys spp. and Mus (Nannomys) spp.; Mus (Nannomys) were
trapped only between July 2010 and February 2011. A small number of African Giant
Shrews Crocidura olivieri (3.3%) were also captured, but were not considered in the
estimation of abundance.
Table 1
Fig. 2
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3.2. Seasonality, crop stage and rodent abundance
Generally, rodent abundance varied seasonally during the study period (Fig. 2a & b). The
seasonal changes in the abundance showed two sets of peaks each year; the highest peaks
occurring early in the dry season (October to January) - hereafter called “early dry season
peak”, and the second peaks occurring in the wet season (July to August) - hereafter
called “wet season peak”. A drop in abundance was observed immediately after the wet
season peaks across the study period (October 2007, 2008, and 2009 and September
2010), followed by a resurgence of abundance in the following months, resulting in the
early dry season peaks. Note that the cropping stages are related to the wet and early dry
season peaks correspond with milky stage and harvest respectively (Fig. 3).
3.3. Rodent abundance, crop damage and yield loss
In the HSBD grids, the mean crop damage was 7.1% ((range: 4.6% (2009) –
10.7% (2007)) and 5.0% ((range: 3.1% (2009) – 8.3% (2010)) at the milky and
maturation crop stages, respectively. In LSBD grids, the mean damage was 3.8% ((range:
2.3 (2009) – 5.6% (2007)) and 4.1% ((range: 2.4% (2009) - 7.7% (2010)) at the milky
and maturation crop stages, respectively.
The generalized mixed model showed that rodent abundance, stone bund density
and crop stage all contributed to explaining the observed variation in crop damage (Table
2). We fitted a final model including rodent abundance, stone bund density, crop stage
and an interaction between crop stage and stone bund density (model 4). The likelihood
ratio test comparing this model versus one without the interaction term (model 5) was
highly significant (X2 = 74.7, df = 1, p < 0.0001). Year was incorporated as a random
effect term to take into account that we measured crop damage and rodent abundance on
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the same grids over a four year period. The intraclass correlation was estimated as 0.06
indicating that the design effect was low.
Table 2
Since our model was build on the logit scale using a binomial distribution to
model the response variable (damage versus no-damage), we summarized the model
outcome using a plot of the already backtransformed coefficients (Fig. 3). The graph
clearly shows a positive correlation between rodent abundance and damage. We found
higher damage estimates in the HSBD grids, both at the milky and maturation stages.
This relationship was stronger during milky stage, however, where the difference in
damage at a density of 40 rodents per h-1 reached 50% between LSBD and HSBD grids.
3.4. Estimated loss
On average, our estimate for number of stems per quadrat was 125 and the seed
weight per panicle was 0.9 g (at 12.2% moisture content on average, about a week before
harvest). The estimated amount of crop loss in HSBD grids, from the average damage at
maturation stage (5.0%), was 225 kg ha-1 (range: 140 - 373 kg ha-1). The estimated loss in
LSBD grids, from the average damage at maturation stage (4.1%), was 180 kg ha-1
(range: 108 - 347 kg ha-1). Taking in to account the price of wheat and currency exchange
rate for 2010 (~750 Birr q-1 and 1 US$ = 14 Birr, respectively), the average loss in HSBD
grids was equivalent to 1,688 Birr (121 US$) (range: 1,046 (75) – 2,801 Birr (200 US$)).
The average loss in LSBD grids was equivalent to 1,350 Birr (96 US$) (range: 810 (58) –
2599 Birr (187 US$)).
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Fig. 3
4.
Discussion
4.1. Species composition
The multimammate rat Mastomys awashensis and the grass rat Arvicanthis dembeensis
were the two dominant rodent species in rainfed crop fields in Tigray highlands. Nyssen
et al. (2007) also reported M. awashensis and another Arvicantis species, A. niloticus, as
the two dominant species in Tigray highlands (accounting for 91% of captures). In
irrigated cereal and vegetable fields in other parts of Tigray, about 130 km North West of
Hagere Selam and at a slightly lower altitude (~2000m), M. erythroleucus and A.
dembeensis were reported as the dominant species (accounting for 93% of captures)
(Gebresilassie et al. 2004). Bekele and Leirs (1997) have also reported the latter two
species, accounted for 87% of captures, in maize fields in central Ethiopia.
4.2. Rodent abundance and stone bunds
The overall proportion of the small mammals captured was significantly higher in HSBD
grids than in LSBD grids. Several reports showed higher rodent abundances correlated
with presence of better vegetation and structural cover (Massawe et al. 2006; Jacob,
2003, 2008; Birkedal et al. 2009). However, at the species level only the abundance of
Arvicanthis dembeensis varied significantly between the two stone bund densities, with
more A. dembeensis in HSBD grids than in LSBD grids. We suggest that this is because
A. dembeensis is a heavier and diurnal species (Challet et al. 2002) which seeks relatively
more cover against potential predators. Our findings are in line with Nyssen et al. (2007),
who also found such differences in species abundance with variation in stone bund
density using data collected from one cropping season (between June to November).
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4.3. Rodent abundance and crop phonological stage
Rodent abundance was more pronounced during the reproductive stages of the crop
(milky and fruiting stages) and around harvest. It appeared that as the crops developed
towards the reproductive stages, so did the availability and quality of food and cover
(crop height) which may have been conducive for rodent population growth. Similar
increases in rodent abundance in the course of crop development and increasing
vegetation cover have been reported by Brown et al. (2007) and Jacob (2008). The
pattern we observed appears to occur with much regularity following crop development,
making decision making for application of control measures somewhat less challenging.
This finding is therefore a very useful input for rodent pest management in the highlands
of Ethiopia.
Surprisingly, we were unable to explain why the rodent abundance dropped at
maturation stage of the crop. Crop maturation usually coincides with the onset of the dry
season, at a time when new individuals might be recruited into the population (Leirs,
1992; Makundi et al. 2009; Masssawe et al. 2011). Asynchronous planting among the
farmers resulted in varied crop stages in the already mosaic fields of the Tigray
highlands. This might encourage rodents to move between neighboring fields with
different crop stages in search of better quality food and could have lowered the
population in the study grids. Predation by avian and mammalian predators may also
have contributed to lower rodent populations at the beginning of the dry season, although
their impact in the current study is unknown. Predator impacts on rodent populations can
be direct or indirect. In the former, predators influence population dynamics by
physically removing individuals, whereas in the latter case the presence of predators
15
induces behavioral or physiological responses on the prey due to perceived risk, reducing
the probability of being captured (Mohr et al., 2003; Vibe-Petersen et al., 2006).
4.4. Crop damage and loss
The generalized mixed model result showed correlation between the proportion of
crop damage and abundance of rodents. We found a significantly higher proportion of
damage in HSBD grids, particularly at milky stage, than in LSBD grids. This was
consistent with the proportion of rodents captured, which was significantly higher in
HSBD grids than LSBD grids. Importantly, the abundance of rodents in the HSBD grids
was not only higher, but also consisted of relatively more Arvicanthis dembeensis. A.
dembeensis is larger and heavier than M. awashensis, and it also consumes more biomass
than M. awashensis; we propose that the combination of these factors was the cause of
the greater crop damage in the HSBD. The presence of stone bunds relatively close to
each other might have lowered the perceived risk of predation and generated better
foraging opportunities for this species. Our result was consistent with that of the farmers’
survey in Hagere Selam in that the same crop stage experienced the most critical damage
(Meheretu et al. 2010). Gebresilassie et al. (2004) also reported intense rodent attacks
during the fruiting stage in irrigated cereal fields in Tigray. The presence of higher
proportion of grain protein, fat and several of the B-vitamins in the germ (seed bud) at
milky stage have been argued to supply the dietary requirement of the rodents (Leirs et al.
1990; 1993; Mutze, 2007; Kumar et al. 2011).
Considering an estimate of 0.5 kg average daily per capita grain consumption per
person in Ethiopia (country wide annual per capita grain consumption was estimated as
16
176 kg (Robinson et al., 2006)), the average loss in HSBD fields could have supported a
family of 5 heads for about 3 months (range: 1.9 – 5 months) and the average loss in
LSBD fields could have supported a family of the same size for about 2.4 months (range:
1.4 – 4.6 months). Hence, the average loss caused by HSBD, about 25% more than the
loss suffered in LSBD, roughly equates to two weeks of extra food. For somebody who
has just enough to eat, two weeks extra food is important; for somebody who has enough
to eat but no money, for instance, to buy textbooks to his children, the equivalent of two
weeks eating is also equality important, but the relative effect is less. Note that these
estimates do not include the losses suffered during the rest of the crop stages.
In conclusion, in terms of pest management, increasing the distance between stone
bunds in crop fields may reduce rodent numbers and ultimately crop damage (at least of
the latter species). The fact that rodent abundances peaked during the reproductive stages
of the crop and around harvest entail serious pest management intervention before these
crop stages are attained.
Acknowledgments
We are grateful to the Flemish Interuniversity Council (VLIR), Belgium, for funding the
Ph.D of Y.M.. We thank N. Hughes for helpful comments and native English proofing,
Departments of Biology in Mekelle (Ethiopia) and Antwerp (Belgium) Universities for
provision of laboratory facilities, and Mekelle University–Institutional University
Cooperation (MU-IUC) project staff for facilitating the fieldwork.
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Figure 1 The approximate position of the city of Hagere Selam ( ), in Dogu'a Temben
district, Tigray province (shaded), Northern Ethiopia.
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Figure 2 Monthly estimates of abundance of M. awashensis (dotted lines) and A.
dembeensis (solid lines) in LSBD (a) and HSBD (b) grids. The bars indicate monthly
mean rainfall.
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Figure 3 Relationship between rodent abundance and crop damage (%) in a mixed wheat
and barely crop at milky (Milk.) and maturation (Mat.) stages in HSBD and LSBD grids.
Table 1. Composition (number and percentage) of small mammal species trapped from
LSBD and HSBD grids in rainfed crop fields around Hagere Selam, Northern Ethiopia,
from April 2007 to February 2011.
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Table 2 The results of the generalized linear models fitted to relate the observed
variation in crop damage (perc. total) to rodent abundance (m(h)), different crop stages
(crop stage), stone bund density (stone bund), and year.
28